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Gravity or Gravitation is a property by which all objects attract each other. Modern physics describes gravitation using the general theory of relativity, but the much simpler Newton's law of universal gravitation provides an excellent approximation in many cases. Gravitation is the reason for the very existence of the Earth, the Sun, and other celestial bodies; without it, matter would not have coalesced into these bodies and life as we know it would not exist. Gravitation is also responsible for keeping the Earth and the other planets in their orbits around the Sun, the Moon in its orbit around the Earth, for the formation of tides, and for various other natural phenomena that we observe. History of gravitational theory Early (pre-Newtonian) history Since the time of the Greek philosopher Aristotle in the 4th century BC, there have been many attempts to understand and explain gravity. Aristotle believed that there was no effect without a cause, and therefore no motion without a force. He hypothesized that everything tried to move towards their proper place in the crystalline spheres of the heavens, and that physical bodies fell toward the center of the Earth in proportion to their weight. Another example of an attempted explanation is that of the Indian astronomer Brahmagupta who, in 628 AD, wrote that "bodies fall towards the earth as it is in the nature of the earth to attract bodies, just as it is in the nature of water to flow". Modern work on gravitational theory began with the work of Galileo Galilei in the late 16th century and early 17th century. In his famous experiment dropping balls at the Tower of Pisa and later with careful measurements of balls rolling down inclines, Galileo showed that gravitation accelerates all objects at the same rate. This was a major departure from the Aristotle's belief that heavier objects are accelerated faster. (Galileo correctly postulated air resistance as the reason that lighter objects appear to fall more slowly.) Galileo's work set the stage for the formulation of Newton's theory of gravity. Newtons theory of gravitation In 1687, English mathematician Sir Isaac Newton published the famous Principia, which hypothesizes the inverse-square law of universal gravitation. In his own words, "I deduced that the forces which keep the planets in their orbs must be reciprocally as the squares of their distances from the centers about which they revolve; and thereby compared the force requisite to keep the Moon in her orb with the force of gravity at the surface of the Earth; and found them answer pretty nearly." Most modern non-relativistic gravitational calculations are based on Newton's work. General relativity Newton's conception and quantification of gravitation held until the beginning of the 20th century, when the German-born physicist Albert Einstein proposed the general theory of relativity. In this theory Einstein proposed that inertial motion occurs when objects are in free-fall instead of when they are at rest with respect to a massive object such as the Earth (as is the case in classical mechanics). The problem is that in flat spacetimes such as those of classical mechanics and special relativity, there is no way that inertial observers can accelerate with respect to each other, as free-falling bodies can do as they are each accelerated towards the center of a massive object. To deal with this difficulty, Einstein proposed that spacetime is curved by the presence of matter, and that free-falling objects are following the geodesics of the spacetime. More specifically, Einstein discovered the field equations of general relativity, which relate the presence of matter and the curvature of spacetime. The Einstein field equations are a set of 10 simultaneous, non-linear, differential equations whose solutions give the components of the metric tensor of spacetime. This metric tensor allows calculation of not only angles and distances between space-time intervals (segments) measured with the coordinates against which the spacetime manifold is being mapped but also the affine-connection from which the curvature is obtained, thereby describing the spacetime's geometrical structure. Notable solutions of the Einstein field equations include: General relativity has enjoyed much success because of how its predictions have been regularly confirmed. For example: Earths gravity
Equations for a falling body Under normal earth-bound conditions, when objects move owing to a constant gravitational force a set of kinematical and dynamical equations describe the resultant trajectories. For example, Newton’s law of gravitation simplifies to F = mg, where m is the mass of the body. This assumption is reasonable for objects falling to earth over the relatively short vertical distances of our everyday experience, but is very much untrue over larger distances, such as spacecraft trajectories, because the acceleration far from the surface of the Earth will not in general be g. A further example is the expression that we use for the calculation of potential energy of a body = mgh. This expression can be used only over small distances from the Earth. Similarly the expression for the maximum height reached by a vertically projected body, is useful for small heights and small initial velocities only. In case of large initial velocities we have to use the principle of conservation of energy to find the maximum height reached. Gravity and astronomy The discovery and application of Newton's law of gravity accounts for the detailed information we have about the planets in our solar system, the mass of the Sun, the distance to stars and even the theory of dark matter. Although we have not traveled to all the planets nor to the Sun, we know their mass. The mass is obtained by applying the laws of gravity to the measured characteristics of the orbit. In space an object maintains its orbit because of the force of gravity acting upon it. Planets orbit stars, stars orbit galactic centers, galaxies orbit a center of mass in clusters, and clusters orbit in superclusters. Gravity versus gravitation It is important to note that gravitation is not gravity. Gravitation is the attractive influence that all objects exert on each other, while "gravity" specifically refers to a force which all massive objects are theorized to exert on each other to cause gravitation. Although these terms are used interchangably in everyday use, it is important to note that in theories other than Newton's, gravitation is caused by factors other than gravity. For example, in general relativity, gravitation is due to spacetime curvatures which causes inertially moving object to tend to accelerate towards each other. Another (but discredited) example is Le Sage's theory of gravitation, in which massive objects are effectively pushed towards each other. Applications
Alternative theories Historical alternative theories Recent alternative theories See also Notes | |||||||||||
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